The interferometric spectrum analyzer is an optical signal processing device in which a pair of separately modulated optical beams, derived from a common light beam source, are combined on a photodetector array, the output of which is proportional to the instantaneous magnitude spectrum .vertline.F.sub.T (.omega.,t).vertline., where F.sub.T (.omega.,t) is the instantaneous spectrum of the input waveform F(t) that has been converted into an optical waveform through an acousto-optic medium, such as an acousto-optic Bragg cell. Conventionally, the separately modulated optical beams are obtained by dividing an optical beam from a coherent light source such as a laser, into two separate paths, as in a conventional Mach-Zehnder scheme shown in FIG. 1.
More particularly, a collimated laser beam 10 from an optical source (e.g. laser) 11 is split by a beam splitter (e.g. half-silvered mirror) 13 into a reference path 21 and a signal path 23. Within the reference path 21, a first acousto-optic beam deflector (e.g. acousto-optic Bragg cell) 25 is driven by an appropriately modulated (e.g. linear frequency) reference signal which produces a deflected beam 27, the angle of deflection of which is proportional to the instantaneous frequency of the signal applied to the Bragg cell 25, and which is incident on a combining mirror 31. The undeflected beam 28 may be removed before or after the combining mirror 31.Within the signal path 23, a second Bragg cell 33 is inserted. An unknown frequency signal of interest to be analyzed is applied as an input to the Bragg cell 33, causing signal beam 23 to be deflected in proportion to the input frequency. The deflected beam 29 interferes with beam 27 through mirror 31 and the combined beams travel along path 35 or an equivalently symmetric other useful path 36.
Downstream of mirror 31, a Fourier transform lens 37 focusses the incident reference and signal beams from mirror 31 onto a photodetector array 41. On the photodetector array 41, located at the Fourier transform plane of lens 37, an interference pattern formed by the Fourier transform combination of the two (Bragg cell-modulated) beams is created. At the Fourier transform plane whereat the photodetector array is located, the deflected reference and signal beams combine to produce a beat frequency representative of the difference between the reference signal applied to Bragg cell 25 and the unknown signal of interest applied to signal Bragg cell 33. Since the input frequency to Bragg cell 25 is known and the spatial location of any beat frequency is frequency dependent, the contents of the unknown signal can be detected by monitoring the output of the photodetector array.
Other embodiments of conventional interferometric acousto-optic spectrum analyzers include the use of Koester prisms and parallel beams that are merged on a photodetector array output. Regardless of the type of system employed, however, the conventional separation/combining mechanism is subject to a number of disadvantages. First of all, because separate beam paths (reference and signal) and components are employed, the system suffers instability due to variations in temperature, vibration, air currents, etc and the problem of maintaining precise alignment of all of the components. In addition, when the signal source beam is separated into respective reference and signal beam paths, there is a loss of optical energy at the beam splitter upstream of the acousto-optic modulation elements, and at the beam combiner (mirror) downstream of the acousto-optic modulation components. This leads to a reduction in system dynamic range. Additionally, the large number of components required to achieve beam path separation and merging necessarily results in an increase in weight, volume and cost of the overall system.